Electrical oscillation generator



g 26, 1941- w. s. PERCIVAL 2,254,087

ELECTRICAL O'SCILLATION GENERATOR Filed Oct. 27, 1958 .9 6 Z 5 l '2 T 4 ,4

INVENTOR WILL/AM SPENCER PERC/VAL BY AT W Patented Aug. 26, 1941 William Spencer Percival,

Ealing, London, England, assignor to Electric 8a Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Appiication October 27, 1938, Serial No. 237,342

In Great Britain November 4, 1937 2 Claims. ,(CL250-36) The present invention relates to electrical oscillation generators of the blocking oscillator type and particularly, but not exclusively, to oscillators for generating substantially saw-tooth oscillations for television purposes.

In a known form of blocking oscillator a condenser is connected to a direct current source through aresistance and is arranged to be shortcircuited by the anode circuit of a thermionic vacuum valve, the control grid and cathode of which are connected through a coil and a resistance shunted by a condensen while another coil coupled with the first mentioned coil isconnected in the screening grid circuit of said valve.

This known form of blocking oscillator when used for the production of substantially sawtooth oscillations for' the scanning in television receivers suffers from certaindisadvantages, particularly in connection with the application In the first thereto of synchronising pulses. place, if an attempt is made to feed the grid circuit of theblocking oscillator directly with television signals, which may consist of a series of picture signals separated by synchronising pulses, then owing to the feedback to the control grid from the screening grid circuit of the blocking oscillator, the signals conveyed to the picture reproducer, as for example, a cathode raytube, whichis assumed to be also connected with the source of television signal, will suffer interference. It is thus essential in the known arrangement to employ a separating valve between the common output of picture signals and synchronising pulses and the blocking oscillator.

Secondly, it is not possible with the known form of blocking oscillator to feed to the control grid thereof signals which include a D. C. component, owing to the presence of the blocking condenser and coupling coil. It is therefore essential in the case of television signals to eliminate the picture signals from the synchronising pulses before applying the latter to the blocking oscillator. If the picture signals are not eliminated and the D. C. component of the picture signals is missing, then the height of the synchronising pulses will depend on the amplitude of the previous picture signals and the synchronising will be erratic, whereas when the D. C. component is present the synchronising pulses will be of constant height.

It is an object of the present invention to provide an improved oscillation generator in which the above disadvantages are reduced or obviated.

According to the invention a blocking oscilfiv condenser after a bias the cathode of said valve to such a potential lator is provided comprising a thermionic valve which is so arranged as 'tobe periodically rendered conducting when controlling potentials V are applied to the control'grid thereof, the oathode circuit of said valve being coupled in a regenerative sense to a further electrode of said valve, other than the control grid, so as to leave said control grid available solely for the application of said controlling potentials, and a block- T ing condenser with a leakage pathis provided in the cathodecircuit of said valve, said blocking condenser being charged when said valve is rendered conducting'the charge accumulated in said predetermined time serving to ing the D. C. component in addition to the synchronising pulses; if the synchronising pulses are applied in a positive sense to the control grid of the valve, the presence ofv the picture a s which will be negative with respect to the synchronising pulses will not upset the synchronising action. This is. because with the D. C. component of the picture signal, present, the maximum potential of the synchronising pulses in a. positive direction will be quite independent of the previous picture.

In order that the said invention may be clearly understood and readily carried into effect the same will now be described with reference to the accompanying drawing which illustrates a blocking oscillator in accordance with the invention for the generation of substantially sawtooth potentials.

In the figure, 2 represents a thermionic valve of the screen-grid type, comprising a cathode 3, a control grid 4, a screening grid 5 and an anode 6. The anode 6 is connected to the junction of a resistance 1, connected by means of lead l5 to the positive pole of a source of direct current, and a condenser l4 connected by means of lead It to the negative pole of the direct current source, which is earthed. The sawtooth shaped potential output is taken from the condenser l4. Between the cathode 3 and the negative pole (earth) of the direct current source is connected a coupling coil ll of a feedback transformer and a resistance l3, which may be variable, shunted by a blocking condenser l2. The time constant of the resistance-capacity circuit l2, l3, determines approximately the frequency of the blocking oscillations. The second coupling coil [B of the feedback transformer is connected at one end to the cathode 3 and at the other end to the screening grid 5 via a condenser 9. The screening grid 5 is supplied with a suitable potential by means of resistance 8 connected to the positive pole of the D. C. source. When used for television purposes the control grid 4 is connected by means of lead I to one terminal of a source of controlling potentials which may consist of positive synchronising pulses and possibly also the accompanying picture signals including the D. C. component. The other terminal of the source of controlling potentials is connected to the earth line l6. If necessary, a fixed positive bias potential may be applied to the grid 4. In operation, when the valve 2 is non-conducting the condenser 14 is charged relatively slowly through the resistance 1 and is discharged when the valve is rendered conducting.

The feedback transformer is so arranged that when the positive synchronising pulse on the control grid 4 causes the valve to conduct oscillations are initiated and the blocking condenser I2 is charged so as to bias the cathode 3 in such a way as to render the valve non-conducting, the resistance I3 serving as a discharge path for the blocking condenser. The valve is thus rendered conducting and insulating periodically and this produces a saw tooth oscillation across condenser M. It will be obvious that as the control grid is not associated with the oscillation circuit substantially no interfering potentials are fed back into the source of controlling signals and also that it is possible to apply the D. C. component of the picture signals to the control grid of the valve.

It will be understood that various arrangements of the above described circuit are possible while still conforming to the statement of the invention. For example, the condenser I2 and resistance I3 may be shunt connected and inserted in the circuit between the cathode 3 and the junction of feedback windings l0 and Ii, the free end of winding ll being connected to the earth line l6. In an alternative modification the resistance l3 may be connected between cathode 3 and the earth line 16. This alternative modification may be modified by connecting condenser I2 between the junction of feedback windings iii and II and the cathode 3, the free end of winding H being connected to the earth line 16.

By making the discharge of condenser I2 sufficiently linear it is possible to derive a sawtooth potential across condenser IZ, this sawtooth potential being in opposite phase to the potential generated across the condenser l4. Thus the circuit may be arranged to'generate sawtooth potentials in pushpull. Various ways of linearising the discharge are possible, for example, an inductance may be inserted in series with the resistance I 3 or the resistance l3 may be taken to a negative potential with respect to earth. In addition, a positive potential may be applied to grid 4.

As a further modification of the embodiment illustrated in the figure it is possible to connect condenser 14 from the anode 6 to the positive source of potential l5. In this case when the valve 2 conducts the condenser 14 will be rapidly charged and when the valve 2 insulates the condenser will be slowly discharged through the resistance 1.

I claim:

1. A serrated wave form generator comprising a thermionic tube having anode, cathode, control and screen electrodes, electric storage means connected in the anode-cathode circuit of said tube, a point of reference potential, a time constant circuit comprising resistance and condenser means, said time constant circuit being electrically connected to said point of reference potential and serially connected between the cathode of said tube and the point of reference potential, means coupling said cathode tosaid screen electrode comprising the-series connection of a first inductance and a condenser, a second inductive member coupled to said series inductance and connected serially between said cathode and the point of reference potential and an input circuit for said tube connected between said point of reference potential and said control electrode.

2. A serrated wave form generator comprising a thermionic tube having anode, cathode, control and screen electrodes, electric storage means connected in the anode-cathode circuit of said tube, a point of reference potential, a time constant circuit comprising resistance and condenser means, said time constant circuit being, electrically connected to said point of reference potential and serially connected between the cathode of said tube and the point of reference potential, means coupling said cathode to said screen electrode, said means having at least a portion thereof connected serially with said time constant circuit between said cathode and said point of reference potential, and an input circuit for said tube connected between. said point of reference potential and said control electrode.

WILLIAM SPENCER PERCIVAL.v 

